The present invention relates to a bonding type semiconductor substrate and a semiconductor light emitting element, and preparation processes thereof. Particularly, the present invention relates to a bonding type semiconductor substrate based on a novel wafer direct bonding technique, a light emitting element such as LEDs (light emitting diode) with a high brightness obtained by applying the technique, and preparation processes thereof.
A light emitting element based on a conventional technique will be explained with reference to drawings attached hereto. FIG. 22 shows an embodiment of a visible light LED of InGaAlP based on a conventional technique.
In an LED 100 shown in FIG. 22, InGaAlP epitaxial growth layers 84, 85, 86 contributing to light emission are formed on an n-type GaAs substrate 82. Although not shown in this drawing, a buffer layer may be disposed between the substrate and the epitaxial growth layer in compliance with required specifications in order to obtain an excellent light emitting layer.
Electrodes 89 for supplying an electric current are each disposed on the upper surface of the epitaxial growth layer 86 and the lower surface of the substrate 82, respectively. Although not shown in the same drawing, a layer for diffusing the electric current or a layer for taking an electric contact is often disposed between an upper electrode 89 and the epitaxial growth layer 86. Among the epitaxial growth layers 84, 85, 86, the layer which can emit light by the recombination of carriers is the active layer 85. The epitaxial layers 84 and 85 formed on and under the active layer 85 are the cladding layers 84 and 86 having a wider band gap than the active layer in order to confine the carriers and to thereby heighten an emission efficiency.
For these epitaxial growth layers 84, 85 and 86, the band gap is required to be optimized according to a design for the purpose of adjusting the wavelength of the emission and for the purpose of confining the carriers. Furthermore, it is desirable for a good epitaxial growth that a lattice constant of the epitaxial growth layer matches a lattice constant of the substrate 82. Since InGaAlP which is a group III-V compound contains three elements of In, Ga and Al as components in the group III, the band gap and the lattice constant can be independently designed by selecting a composition ratio of these elements.
For example, when a composition of the epitaxial growth layer is represented by the following formula
Inx(Ga1xe2x88x92yAly)1xe2x88x92xPxe2x80x83xe2x80x83(1),
the lattice constant of the epitaxial growth layer almost matches that of a GaAs substrate by setting a composition ratio (x) of In to 0.5. The band gap can be controlled by adjusting a composition ratio (y) between Al and Ga, while x=0.5 is kept up.
For example, in order to obtain a red light emitting LED having a wavelength of 644 nm, the composition ratio of the active layer 85 is set to x=0.5 and y=0.043, and the composition ratio of the cladding layers 84, 86 is set to x=0.5 and y=0.7. Moreover, in order to obtain a green light emitting LED having a wavelength of 562 nm, the composition ratio of the active layer 85 is set to x=0.5 and y=0.454, and the composition ratio of the cladding layers 84, 86 is set to x=0.5 and y=1.00, i.e., InAlP.
As illustrated in the above, for InGaAlP epitaxial growth layers, the wavelength of the emission can be selected within a region of visible light. Furthermore, since the layers can epitaxially grow under the lattice matching condition with a GaAs substrate which is most general as a compound semiconductor substrate, there exist advantages that the substrate is easily available and the epitaxial growth is relatively easy.
On the contrary, a GaAs substrate has a disadvantage that it absorbs light in a region of visible light. Therefore, part of the light emitted on an InGaAlP epitaxial growth layer is absorbed by GaAs substrate, and hence a brightness of LED decreases unavoidably. For avoiding the decrease of the brightness, a material which is transparent in the region of the visible light is preferably employed as a substrate. The usual transparent material is GaP, but since the GaP substrate cannot obtain the lattice matching with InGaAlP, the good epitaxial growth is difficult. In order to solve this problem, U.S. Pat. No. 5,376,580 filed in 1993 proposes a method for a wafer bonding between an InGaAlP epitaxial growth layer and a GaP substrate. This proposed method comprises removing a GaAs substrate from the epitaxial growth layer, closely bonding a GaP substrate instead of the GaAs substrate, and then carrying out a heat treatment under pressure to integrally bond them. According to this method, the increase of the brightness of LED can be attained. However, the stability and productivity of the wafer bonding step are insufficient, because the epitaxial growth layer after the removal of the GaAs substrate is thin and thus its handling is difficult, and a special apparatus is necessary for the heat treatment under pressure.
Next, the following explains the wafer bonding. If two different kinds of wafers can be integrally bonded, a laminated structure comprising different materials could be obtained regardless of the lattice constant, and a different substance can be buried inside as represented by SOI (silicon on insulator). For these reasons, various wafer bonding techniques have been proposed hitherto. For example, the above-mentioned bonding method of subjecting two wafers to the heat treatment while they are pressed is disclosed in Japanese Patent No. 765892 filed in 1970. The wafer bonding technique has been desired for a long time, but it is difficult to accomplish the integral bonding all over the surface of the wafer, so that this technique has not been practiced.
The present inventors have developed a technique called xe2x80x9cdirect bondingxe2x80x9d or xe2x80x9cdirect joiningxe2x80x9d as a practicable technique. For example, Japanese Patent No. 1420109 filed in 1983 and the like describes the direct bonding between Si wafers, and Japanese Patent No. 2040637 filed in 1985 and the like describes the direct bonding between compound semiconductor wafers.
The direct bonding technique comprises mutually closely bonding two substrates having mirror-finished surfaces by themselves at room temperature under a substantially dust-free atmosphere, and then integrally joining them by a heat treatment. This technique has an advantage that the whole surface can be joined without leaving any unbonded part, because the whole surfaces are closely bonded to each other prior to the heat treatment. Moreover, it is not necessary to apply a pressure during the heat treatment, any special apparatus or equipment is not required. The mechanism of the direct bonding between the Si wafers is understood as follows.
Namely, at the beginning, OH groups are formed on the surface of the wafer by cleaning or washing with water. Then, when the surfaces of the two wafers are contacted with each other, the OH groups attract each other by a hydrogen bond, so that the wafers closely bond at room temperature. A bonding power in this case is strong enough to eliminate a usual warp of the wafer, whereby the close bonding all over the surfaces can be achieved. During the heat treatment, a dehydrative condensation (Sixe2x80x94OH: HOxe2x80x94Sixe2x86x92Sixe2x80x94Oxe2x80x94Si+H2O) occurs at a temperature higher than 100xc2x0 C., and the wafers bond to each other via oxygen atoms, thereby increasing a bonding strength. When the temperature further rises, the diffusion and rearrangement of the atoms in the vicinity of a bonding interface occur, so that the wafers are integrated mechanically and electrically. The bonding mechanism of the compound semiconductors is considered to be similar.
Next, one example of a preparation process of an LED comprising an InGaAlP-based epitaxial growth layer closely bonded to the GaP substrate by utilizing the direct bonding will be explained with reference to FIG. 23.
First, as shown in FIG. 23A, an n-type cladding layer 94, an active layer 95 and a p-type cladding layer 96 are grown on an n-type GaAs substrate 92. Then, as shown in FIG. 23B, a GaP substrate 91 is directly bonded to the surface of the epitaxial growth layer 96. Furthermore, shown in FIG. 23C, the GaAs substrate 92 is removed by polishing or etching, and after upside down, electrodes 99 are disposed on the upper surface of the n-type cladding layer 94 and under the lower surface of GaP substrate 91, respectively, so that an InGaAlP-based LED having GaP as the substrate 91 is obtained as shown in FIG. 23D.
In the case that the different materials are directly bonded to each other in such a manner, especially in the case that the surfaces of the epitaxial growth layers are directly bonded, the following problems exist, in contrast to the direct bonding of the same wafers such as Si wafers or GaAs wafers.
First, on the surface of the epitaxial growth layer, particles such as dust are more easily deposited than on the surface of the wafer. Accordingly, the bonding at room temperature is hindered, and even after the heat treatment, the boding is not accomplished all over the surface and hence there is a problem that unbonded portion called xe2x80x9cvoidsxe2x80x9d occur. Generally, the surface of a wafer is kept clean and a clean substrate is employed as a substrate for epitaxial growth. However, particle adhesion on the surface of the epitaxial growth layer is inevitable to some extent at present, since reaction products are deposited during the epitaxial growth and dust particles adhere in a pretreatment step or a post-treatment step of the epitaxial growth step.
Second, since the wafer is warped by the epitaxial growth, there is a problem that it is impossible to bond the whole surface of the wafers closely at room temperature.
Third, since difference of thermal expansion exists between different materials, there is a problem that thermal stress occurs during the heat treatment, and thereby the bonded substrate is broken by the stress.
Fourth, since difference of thermal expansion exists between different materials, a gap at the bonding interface occurs during the heat treatment for bonding even if the breakage of the bonded substrate does not occur, and thus the whole surface of the substrate cannot be bonded homogeneously by xe2x80x9cthe gapxe2x80x9d.
Fifth, there is a problem that electric resistance is generated at the bonding interface. The original investigation of the present inventors discloses that an electrically resistive component is generated by bonding the wafers. When LED, for example, is formed using such a bonded substrate, the electric resistance at the bonding interface increases operating voltage of LED and thus problems such as a poor emission and a heat generation occur.
The present invention has been accomplished in view of the above circumstances, and an object of the present invention is to provide a bonding type semiconductor substrate directly, stably and closely bonded all over the surface of an epitaxial growth layer formed on a semiconductor substrate.
Another object of the present invention is to provide a semiconductor light emitting element.
Still another object of the present invention is to provide a preparation process of the above substrate.
A further object of the present invention is to provide a preparation process of the above light emitting element.
In the present invention, the above-mentioned objects can be achieved by the following constitutions.
According to the present invention, there is provided a bonding type semiconductor substrate comprising a first epitaxial growth layer formed on a first semiconductor substrate, and a second semiconductor substrate whose at least one surface is mirror-finished and which is integrally bonded to the first epitaxial growth layer via the mirror-finished surface or a second epitaxial growth layer grown on the mirror-finished surface; a thermal expansion coefficient of the first epitaxial growth layer being close to a thermal expansion coefficient of the second semiconductor substrate.
Since the thermal expansion coefficient of the first epitaxial growth layer is close to the thermal expansion coefficient of the second semiconductor substrate, the bonded semiconductor substrate is not broken by the generation of thermal stress, even when a heat treatment is carried out prior to the removal of the first semiconductor substrate. Accordingly, the bonding type semiconductor substrate stably and closely bonded can be provided.
According to the present invention, there is also provided a semiconductor light emitting element comprising:
a first epitaxial growth layer formed from mixed crystals of a compound semiconductor on a first semiconductor substrate, said first epitaxial growth layer including a first cladding layer formed by selecting a composition ratio of the mixed crystals so as to match a lattice constant of said first semiconductor substrate, an active layer formed on said first cladding layer, and a second cladding layer formed on said active layer; said first semiconductor substrate being removed from the first epitaxial growth layer;
a second semiconductor substrate whose a main surface is mirror-finished and which is directly bonded to said first epitaxial growth layer via the mirror-finished surface or a second epitaxial growth layer grown on the mirror-finished surface.
Since the lattice constant of the first epitaxial growth layer matches that of the first semiconductor substrate, the warp of a wafer having the epitaxial growth layer (hereinafter, referred to as xe2x80x9cepi-waferxe2x80x9d) can be reduced. Accordingly, there can be provided the bonding type semiconductor substrate where even the relatively thick epi-wafer is stably and closely joined to the second semiconductor substrate.
The first semiconductor substrate is formed from GaAs, the second semiconductor substrate is formed from GaP, and the epitaxial growth layer is represented by the formula Inx(Ga1xe2x88x92yAly)1xe2x88x92xP. A composition ratio in the above composition formula of the first cladding layer is suitably 0.45 less than x less than 0.50 and 0xe2x89xa6yxe2x89xa61.
According to the invention, there provided a process for preparing a bonding type semiconductor substrate comprising:
a first step of forming a first epitaxial growth layer by epitaxially growing semiconductor crystals on a first semiconductor substrate,
a second step of removing contaminants and dust particles from the surface of said first epitaxial growth layer, and
a third step of integrally joining, to said first epitaxial growth layer from which the contaminants and the dust particles are removed in the second step, a second semiconductor substrate whose at least one surface is mirror-finished, by placing the substrate on the first epitaxial growth layer so that the substrate may come into contact with the first epitaxial growth layer via the mirror-finished surface or a second epitaxial growth layer grown on the mirror-finished surface.
Since particles deposited onto the surface of the epitaxial growth layer can be removed in the second step, the second semiconductor substrate can be joined to the epitaxial growth layer. Accordingly, the semiconductor substrate can be prepared in a high yield.
According to the invention, there also provided a process for preparing a bonding type semiconductor substrate comprising:
a first step of forming an epitaxial growth layer by epitaxially growing semiconductor crystals on a first semiconductor substrate by selecting a composition ratio so as to match a lattice constant of the first semiconductor substrate,
a second step of removing contaminants and dust particles from the surface of said epitaxial growth layer, and
a third step of integrally joining a second semiconductor substrate, whose at least one surface is mirror-finished, to said epitaxial growth layer by placing the substrate on the epitaxial growth layer so that the substrate may come into contact with the epitaxial growth layer via the mirror-finished surface.
Since the lattice constant of the epitaxial growth layer matches that of the first semiconductor substrate, the warp of the epi-wafer can be reduced. Accordingly, the epi-wafer can be stably joined to the second semiconductor substrate, so that the bonding type semiconductor substrate can be prepared in a higher yield.
The process may further comprising a forth step of subjecting the resultant laminate to a heat treatment after at least part of the surface of said first semiconductor substrate has been removed therefrom.
In the above fourth step, at least the surface part of the first semiconductor substrate is removed prior to the heat treatment, and hence an average thermal expansion coefficient of the whole epi-wafer is much the same as the thermal expansion coefficient of the epitaxial growth layer and is close to the thermal expansion coefficient of the second semiconductor substrate. Accordingly, the breakage of the bonded substrate at the successively repeated heat treatment can be prevented, so that the joining type semiconductor substrate which is more excellent in bonding strength can be prepared in a much higher yield.
Moreover, the above first step preferably contains a step of forming a cover layer on the epitaxial growth layer, and the above second step is desirably a step of removing the cover layer by etching. Accordingly, it is unnecessary to directly remove the surface part of the epitaxial growth layer, whereby it becomes possible to precisely control the thickness of the epitaxial growth layer.
According to the invention, there also provided a process for preparing a semiconductor light emitting element comprising:
a step of forming a first epitaxial growth layer comprising a laminate where a first cladding layer, an active layer, and a second cladding layer are deposited in turn on a first semiconductor substrate by growing mixed crystals of a compound semiconductor,
a step of forming a cover layer on said first epitaxial growth layer,
a step of exposing a surface of said first epitaxial growth layer by removing said cover layer,
a step of integrally joining a second semiconductor substrate having a mirror-finished main surface to said first epitaxial growth layer having the exposed surface by placing the substrate on said first epitaxial growth layer so that said main surface of the substrate may come into contact with said first epitaxial growth layer,
a step of subjecting the laminate to a heat treatment at a temperature below a temperature at which the bonding surface is broken owing to a difference between thermal expansion coefficients of said first semiconductor substrate and said second semiconductor substrate,
a step of exposing said first epitaxial growth layer by removing said first semiconductor substrate, and
a step of forming electrodes on the front side of said first epitaxial growth layer and on the back side of said second semiconductor substrate.
The above first cladding layer is desirably formed by selecting a composition ratio of the mixed crystals so that the lattice constant may match the lattice constant of the first semiconductor substrate.
According to the invention, there also provided a process for preparing a semiconductor light emitting element comprising:
a step of forming an epitaxial growth layer comprising a laminate where a first cladding layer, an active layer, and a second cladding layer are deposited in turn on a first semiconductor substrate by epitaxially growing mixed crystals of a compound semiconductor,
a step of forming a cover layer on said second cladding layer,
a step of exposing the surface of said second cladding layer by removing said cover layer,
a step of integrally bonding a second semiconductor substrate having a mirror-finished main surface to said second cladding layer having the exposed surface by placing the substrate on said second cladding layer so that said main surface of the substrate may come into contact with said second cladding layer,
a step of carrying out a heat treatment after at least a surface part of said first semiconductor substrate has been removed by etching,
a step of exposing said first cladding layer by etching, and
a step of forming electrodes on the front side of said first cladding layer and on the back side of said second semiconductor substrate.
The epitaxial growth layer is formed between the first semiconductor substrate and the laminate, and preferably contains a protective film which functions as an etching stopper in the step of removing the first semiconductor substrate. This protective film can afford a margin at the time of the etching and can also prevent the evaporation of doped impurities and P (phosphorus) which is a constitutional component of the first cladding layer in the step of the above heat treatment.
According to the another aspect of the invention, crystals are aligned in a substantially similar direction, and hence a dangling bond and crystal defects can be reduced and the rise of electric resistance can be inhibited.
More specifically, there provided a bonding type semiconductor substrate comprising:
a first epitaxial growth layer formed on a first semiconductor substrate; and
a second semiconductor substrate whose at least one surface is mirror-finished and which is integrally joined to said first epitaxial growth layer via the mirror-finished surface or a second epitaxial growth layer grown on the mirror-finished surface,
both of said first semiconductor substrate and said second semiconductor substrate being made from compounds,
a surface where a (111) A plane preferentially appears in the main surface of the second semiconductor substrate being bonded to a surface where a (111) B plane preferentially appears in the main surface of said first semiconductor substrate, or a surface where a (111) B plane preferentially appears in the main surface of the second semiconductor substrate being bonded to a surface where a (111) A plane preferentially appears in the main surface of said first semiconductor substrate
There also provided a semiconductor light emitting element comprising a remaining part of a bonding type semiconductor substrate,
said bonding type semiconductor substrate having:
a first epitaxial growth layer formed on a first semiconductor substrate; and
a second semiconductor substrate whose at least one surface is mirror-finished and which is integrally joined to said first epitaxial growth layer via the mirror-finished surface or a second epitaxial growth layer grown on the mirror-finished surface,
both of said first semiconductor substrate and said second semiconductor substrate being made from compounds,
a surface where a (111) A plane preferentially appears in the main surface of the second semiconductor substrate being bonded to a surface where a (111) B plane preferentially appears in the main surface of said first semiconductor substrate, or a surface where a (111) B plane preferentially appears in the main surface of the second semiconductor substrate being bonded to a surface where a (111) A plane preferentially appears in the main surface of said first semiconductor substrate
said remaining part being obtained by removing at least part of said first semiconductor substrate from said bonding type semiconductor substrate.
According to another aspect on the invention, there provided a semiconductor light emitting element comprising a light emitting layer and a substrate having a transparency to emission, wherein
an area of said light emitting layer on said substrate is smaller than an area of said substrate.